The design instructions and design regulations concerning the safety circuits of conveying systems, more particularly of elevators, are changing, as a result of, among other things, developments in microprocessor technology and software technology. As a consequence of new design instructions and design regulations, electronic safety circuits based on microprocessor control and on data bus architecture can be designed. A number of sensors measuring the operation of an elevator, for example, can be connected to the same data bus, and the operation of an elevator can be monitored with a separate electronic supervision unit connected to the data bus.
The purpose of the safety circuit of an elevator is to ensure the safe operation of the elevator in all operating situations. In this connection, also, in the revised EN 81 elevator standard at least a SIL 2 or SIL 3 safety integrity level is required of many basic elements of electronic safety circuits such as of microprocessor-controlled supervision units, data buses, sensors, measuring cables, et cetera. For achieving an adequate safety level, the aforementioned basic elements must often be duplicated; duplication, on the other hand, increases the amount of components, circuits and wiring needed.
As circuits and wiring increase and become more complex, the risk of a connection error or of a wiring error, of connection of the wrong voltage to conductors, et cetera, also increases. The operating voltage for devices to be connected is also often carried in the same cable as the signal conductors. A breakage of the insulation of a conductor might cause a short-circuit or the connection of operating voltage to a signal conductor. Owing to the aforementioned reasons, among others, there is a need for simpler and more reliable electronic safety circuits than those now known in the art.